Effect of precursor chemistry on residual stress of ALD Al₂O₃ and TiO₂ films

Deposited thin films carry residual stress that can be classified into two categories. One is thermal stress that results from differences in thermal expansion coefficients between the substrate and the film, and the other is so called intrinsic stress that is a result of the deposition process itself. Stress can be either compressive or tensile. In extreme case, films under excessive stress can lead to cracking, buckling or delamination of the film. In polycrystalline films, the residual intrinsic stress originates from morphology, such as grain coalescence, crystallization and phase transformation, and from impurities and voids, and development of these parameters during the deposition process. In amorphous films, residual stress also occurs but its sources are more difficult to elucidate. Residual stress of ALD films has been studied for example for Al2O3, TiO2, and the hybrid material alucone.1-4 These studies have focused on effects of deposition temperature, pulsing parameters, film thickness, and post-deposition annealing temperature. In the present paper we report results on how the precursor chemistry affects the residual stress of ALD Al2O3 and TiO2 films deposited at 250 °C to 50 nm thickness onto 150 mm Si wafers. We employed several metal precursors, such as AlMe3, AlCl3, TiCl4 and Ti(OiPr)4 as well as different oxygen sources (H2O, O3, CH3OH) and studied the effect of both the metal precursor and oxygen source on the residual stress of the deposited films. In addition to stress measurements, film characterization was done by X-ray diffraction, scanning electron microscopy and Time-of-Flight Elastic Recoil Detection Analysis for fundamental morphological and compositional properties. Characterization data will be discussed with the residual stress results. Notably large differences in the residual stress were found as a function of precursor chemistry, for both Al2O3 and TiO2.